There is a monitoring apparatus for an energy storage device (secondary battery) electrically connected to a load via a switch (see Patent Document 1 and the like). The monitoring apparatus is supplied with power from the secondary battery to acquire voltage of the secondary battery. The monitoring apparatus repetitively executes monitoring operation of bringing the switch from a closed state into an opened state when a value of the acquired voltage of the secondary battery becomes lower than a predetermined lower limit voltage value, and then bringing the switch from the opened state into the closed state when the voltage value of the secondary battery becomes higher than the lower limit voltage value. When the voltage value of the secondary battery becomes approximate to the lower limit voltage value, the monitoring apparatus shortens an interval of monitoring operation to promptly detect an overdischarging state of the secondary battery and bring the switch into the opened state for protection of the secondary battery.